In an emergency situation, a person in distress typically calls a pre-determined telephone number (e.g., 911 in the United States) to obtain assistance with the emergency from a public safety agency (e.g., law enforcement, fire department, medical unit, animal control, etc.). An emergency call dispatcher located in a public safety answer point (PSAP) facility typically receives the emergency telephone call and subsequently contacts the appropriate public safety agency or agencies to assist in the emergency situation.
In addition to receiving the emergency call, the dispatcher receives by way of a computer console information related to the call. For example, such information may include the telephone number of the telephone from which the emergency call was made, the address from where the call was placed if the call originated from a land line telephone, person's name associated with the telephone, etc. The dispatcher uses this information to assist the public safety agency or agencies in providing the emergency assistance.
When an emergency call originates from a wireless telephone (e.g., a cellular telephone), information related to the location of the wireless telephone needs to be provided to the emergency call dispatcher. In some cases, the wireless telephone includes a global positioning system (GPS) receiver to determine the location of the telephone. Such telephone provides the GPS-derived location to the dispatcher when an emergency call is made. The GPS technique, however, does have some drawbacks. For instance, the GPS receiver requires time to “warm up”, and often takes a relatively long time to determine the location of the caller. Also, there are a number of environments where the GPS receiver is not capable of determining the location of the caller due to, for example, obstructions and/or lack of receiving sufficient number of the GPS satellite signals.
In cases where the wireless telephone does not have GPS capability, the location information must be derived by other techniques. One such technique is employing a triangulation location algorithm using timing information related to when a wireless telephone signal is received at three or more base stations. This technique works for any wireless telephones including those that have GPS receivers and those that do not. The following example illustrates the triangulation technique.
FIG. 1 illustrates a block diagram of an exemplary wireless communication system 100 comprising a carrier network 102 having a mobile switching center (MSC) 104 and other network equipment (not shown), a mobile positioning center (MPC) 106, a plurality of base stations (BS) 108, 110, and 112 located in a densely-populated geographical area 114, and another plurality of base stations (BS) 130 and 132 located in a sparsely-populated geographical area 134. The base stations (BS) 108, 110, 112, 130, and 132 respectively include position determining equipment (PDE) 116, 118, 120, 136, and 138. In this example, the wireless communication system 100 provides wireless communication services to a mobile communication unit (MU) 122 (e.g., a cellular telephone) located in the densely-populated area 114, and to an MU 140 located in the sparsely-populated area 134. In an emergency call situation, the MSC 104 connects an MU to a public safety answer point (PSAP) 150 to assist the caller with the on-going emergency.
In order to provide the PSAP 150 information related to the location of the calling MU in an emergency situation, the wireless communication system 100 uses triangulation of the signal received in at least three (3) base stations (BS). In this regard, each of the base stations (BS) uses the corresponding PDE to time stamp the signal it receives from the calling MU. Each PDE sends the time stamp information to the MPC 106 by way of the MSC 104. The MPC 106 uses the time stamp information to determine the approximate location of the calling MU.
As an example, in the densely-populated area 114, the person using MU 122 initiates an emergency call. In response, the MU 122 sends an emergency call request to the MSC 104 by way of its assigned base station (BS). In this example, the assigned base station is base station (BS) 108. The emergency call request is subsequently processed by the MSC 104 to establish a voice communication link between the MU 122 and the PSAP 150. When the assigned base station (BS) 108 initially receives the emergency call request, the corresponding PDE 116 time stamps the request with a time of t1. Although the other base stations (BS) 110 and 112 are not currently assigned to handle calls for MU 122, they still receive the broadcast containing the emergency call request at different times t2 and t3 (because they are at different distances from the MU 122), which are time stamped by the corresponding PDEs 118 and 120.
The PDEs 116, 118, and 120 send the time stamp information (t1, t2, t3) to the MPC 106 by way of the MSC 104. Using a triangulation algorithm, the MPC 106 determines the approximate location of the MU 122 using the time stamp information (t1, t2, t3). The MPC 106 then sends the location information to the PSAP 150. The MPC 106 needs at least three (3) time stamps to perform the triangulation in order to accurately determine the approximate location of the calling MU. If the MPC 106 receives less than three (3) time stamps, it cannot accurately determine the approximate location of the calling MU. This presents a problem for a sparsely-populated area.
In a sparsely-populated area, there are typically less base stations (BS) per a given area than in a densely-populated area. As a result, there may not exist the required number of base stations (BS) to receive a signal from an MU in order to perform a triangulation to determine the approximate location of the calling MU. For example, in the sparsely-populated area 134, there are only two (2) base stations (BS) 130 and 132. Accordingly, when the MU 140 sends an emergency call request, the corresponding PDEs 136 and 138 in the respective base stations 130 and 132 can only time stamp the request with times t4 and t5, respectively. The PDEs 136 and 138 send the time stamp information t4 and t5 to the MPC 106 by way of the MSC 104. However, in this example, the MPC 106 does not receive the minimum number of time stamp information to perform a triangulation in order to accurately locate the calling MU 140.